Short answer: the top quark is the heaviest known elementary particle. It weighs about 172.5 GeV/c², which is close to the weight of a whole gold atom, even though it is a single particle. It was found in 1995 at Fermilab near Chicago. It lives for only about half of a trillionth of a trillionth of a second, so it breaks down before it can join with other quarks.
Quick facts
- Electric charge: plus two thirds.
- Weight: about 172.5 GeV/c². A joint ATLAS and CMS measurement in March 2024 gave 172.52 with an error of 0.33.
- Lifetime: about 5 × 10-25 seconds.
- Found by: the CDF and DØ teams at the Tevatron collider at Fermilab. The discovery was announced on March 2, 1995.
- Most common decay: it turns into a bottom quark almost every time (about 99.8 percent of the time).
Why was it hard to find?
Scientists expected a sixth quark long before 1995. In 1973, two Japanese physicists, Makoto Kobayashi and Toshihide Maskawa, showed that a third pair of quarks would explain some odd particle behaviour. The bottom quark turned up in 1977, so everyone knew its partner, the top, had to exist. But it was so heavy that only a very powerful collider could make one. It took almost two more decades to build a machine strong enough.
How heavy is “heavy”?
Compare it with the other quarks. See our list of the six quark flavours.
- Up and down: only a few thousandths of a GeV (very light).
- Strange: around 0.1 GeV.
- Charm: around 1.3 GeV.
- Bottom: around 4.2 GeV.
- Top: about 172.5 GeV, which is roughly 40 times heavier than the bottom quark.
No one knows why the top quark is so much heavier than the others. It is one of the open questions in physics.
Why it never forms a “normal” particle
Other quarks are stuck inside particles like protons, because the strong force pulls them into groups. A top quark lives so briefly that it falls apart before that can happen. Scientists say it “decays before it hadronizes”. This makes it special: it is the only quark we can study almost as a free quark, before the strong force changes it.
Why do scientists study it?
- Testing the Standard Model. The top quark’s mass is linked to the mass of the Higgs boson. Careful measurements help check whether our best theory holds together.
- Strong link to the Higgs. The top quark interacts with the Higgs field more strongly than any other particle.
- Clues about new physics. Any small mismatch could point to particles we have not found yet.
New: signs of “toponium” (2025 and 2026)
For years, scientists thought a top quark and its opposite, the top antiquark, would never have time to form a pair, because each one disappears so fast. The same idea works for lighter quarks: a charm quark and an anticharm quark make a particle called charmonium (found in 1974), and a bottom and antibottom make bottomonium (found in 1977).
In 2025, the CMS team at CERN’s Large Hadron Collider reported an unexpected extra number of top-antitop pairs made at the lowest possible energy. The ATLAS team reported a matching result. Both teams say the effect is strong enough to count as a discovery (above 5 sigma). Many physicists read this as a short-lived, “quasi-bound” top-antitop state, nicknamed toponium. Others are still studying what it means, and results in 2026 are adding more detail.
If the toponium idea holds, it will give a new and very precise way to measure the top quark’s mass.
Quick FAQ
Is the top quark the heaviest particle?
It is the heaviest known elementary particle. Bigger particles exist, but they are made of smaller pieces.
Can we see a top quark?
No. It lasts too briefly. Scientists detect the particles it turns into.
Is a top quark inside my body?
No. Only up and down quarks are in everyday matter. See quark vs atom.
Keep reading: what is smaller than a quark and quark-gluon plasma. Back to the science section